The present invention is directed to resilient battery separator media and, in a preferred embodiment, to resilient glass microfiber battery separator media that is especially suited for use as a separator in a battery such as, a starved electrolyte battery.
Rechargeable batteries, such as sealed, starved electrolyte, lead/acid batteries, are commonly used as power sources in vehicles, aircraft, emergency equipment and the like. These batteries, which typically range in size from "D" or "beer can" sized batteries to larger sized batteries, are either single or multi-cell batteries. Currently, each cell of a single cell or multi-cell starved electrolyte, lead/acid battery is defined by a sealed compartment which houses a cell pack that includes at least one porous, positive electrode plate, at least one porous, negative electrode plate, and at least one porous, relatively fragile, glass microfiber separator between the electrode plates. A sulfuric acid electrolyte within each cell is absorbed by the porous, glass microfiber separator(s) and the porous electrode plates. Thus, the separators used in starved electrolyte, lead/acid batteries are intended to function: as separators between the positive and negative electrode plates of the cells to maintain the spacings between the positive and negative electrode plates and prevent the formation of short circuits within the cells; and as reservoirs for retaining electrolyte within the cells between the positive and negative electrode plates.
Short circuits within the cells can occur due to direct contact between the positive and negative electrode plates when the spacings between the electrode plates are not maintained or due to the formation of dendrites or moss shaped particles of the electrode materials between the positive and negative electrode plates. Over the service life of such batteries, the electrode plates repeatedly expand and contract due to changes in active material morphology and density produced by the chemical reactions within the cells producing the electrical energy. Thus, to maintain the spacings between the positive and negative electrode plates over the service life of such a battery and to prolong the service life of such a battery, the electrolyte carrying separators should be resilient to maintain contact with the electrode plates and prevent short circuits within the battery through plate to plate contact. In addition, the separators should be free of openings, formed in the separators either during their manufacture or through the handling of the separators and assembly of the battery cells, to prevent or inhibit the formation of short circuiting active material growths, sheddings or dendrites between the electrode plates through such openings over the service life of the batteries. Thus, the separators should be free or essentially free of openings through which active material growths or dendrites could easily form between the electrode plates and should have sufficient integrity to resist tearing, during the handling of the separators and the assembly of separators into the battery cells, which could form openings in the separators through which active material growths or dendrites could easily form between the electrode plates.
Since the separators in starved electrolyte batteries, such as starved electrolyte lead/acid batteries, also function as electrolyte reservoirs, the capacity of such batteries is a function of both the porosity and surface areas of the electrode plates and the porosity and surface areas of the separators in contact with the surfaces of the electrode plates. Thus, to maintain the electrolyte between the positive and negative electrode plates and to maintain the major surfaces of the separators in contact with the surfaces of the electrode plates, the separators of such batteries should be resilient so that the separators continue to recover in thickness after the repeated expansion and contraction of the electrode plates over the service life of such batteries.
Currently, thin, light weight mats or papers of glass fibers, polymeric fibers and/or other fibers (e.g. mats or papers ranging from about 100 to about 450 grams per square meter, such as glass microfiber separator mats for batteries) are made in various wet laid processes. In these wet laid paper making processes, the fibers are manufactured by various processes and collected in bulk. Batches of the glass and/or polymeric fibers, having more than one fiber diameter, are then introduced into a fiber preparation tank or mixer and dispersed in a water slurry within the tank or mixer which is stirred or agitated to cause the different diameter fibers to become thoroughly and randomly mixed with each other. While the stirring or agitation of the fibers in the preparation tank thoroughly mixes the different diameter fibers together in the slurry, the stirring or agitation of the fibers reduces the length of the fibers and adversely affects the resiliency of the matted papers formed from the slurry. The slurry of mixed fibers is then pumped from the preparation tank or mixer to a conventional paper making screen, e.g. the screen of a Fourdrinier or a Rotoformer machine, and deposited onto the screen where the water is removed from the suspension by suction through the screen to form a wet laid fibrous matted paper on the screen. When intended for use as a battery separator, the matted paper is then processed through an acid bath to bond the fibers of the matted paper together. After the matted paper is formed and, in the case of battery separators, processed through an acid bath, the matted paper is dried and wound up into a roll or otherwise collected in a conventional manner for further processing, such as being cut into selected sizes for use as a battery separator.
These processes for forming thin, light weight matted paper, are costly and result in matted paper products which, due to the relatively short lengths of the fibers in the matted paper, caused at least in part by the stirring or agitation of the fibrous slurry in the preparation tank, exhibit only limited recovery after compression and low integrity. These matted paper products may also have openings through which active material growths or dendrites can form when these products are used as battery separators. Thus, for many end uses and, especially, for uses as separators for starved electrolyte batteries, such as starved electrolyte lead/acid batteries, the relatively high cost of such matted paper products and the limited recovery after compression, limited integrity and limited ability to prevent the formation of active material growths or dendrites exhibited by such products has resulted in the need for a low cost, resilient mat of relatively high integrity that is free or essentially free of openings through which active material growths or dendrites can easily pass between electrode plates.